This invention relates to an enhanced method and device for the isomerization of gasoline with a high benzene content.
It is known that the oil industry resorts to methods that are aimed at increasing the octane number of these gasolines by transforming the straight paraffins they contain into branched paraffins (or isoparaffins). In general, the charge to be treated is composed mostly of saturated hydrocarbons with five or six carbon atoms, as well as lower percentages of hydrocarbons with four or seven carbon atoms, and of benzene that, as we all know, is relatively difficult to separate from the other hydrocarbons with six carbon atoms.
In order to carry out the isomerization reaction, the charge to be treated is most often mixed with hydrogen and a possible recycle, it is then directed toward at least one reactor containing an appropriate fixed-bed catalyst. The temperature in this reactor usually ranges between 120 and 190° C. When exiting the isomerization reactor (or reactors), the effluents are carried toward one or several separation columns. Often times, the isoparaffins are then separated from the non isomerized paraffins: in principle, the first are sent to the gasoline pool, whereas the second are possibly recycled in the reactor, in order to be transformed therein.
We also know that, under isomerization reaction conditions, the benzene that is present in the charge is hydrogenated, due to the presence of the hydrogen and the hydrogenating transition metals that enter into the composition of the isomerization catalysts. This results in a significant heat release in the upstream part of the isomerization reactor due to the exothermicity of this reaction, which is detrimental to the efficiency of the isomerization reaction.
This is why we generally use two isomerization reactors: the first reactor, in which, in addition to the isomerization of paraffins, the hydrogenation of traces of benzene present in the charge occurs, operates at a temperature slightly higher that the second, where the actual isomerization reaction is finished. Indeed, from a kinetic point of view, the isomerization reaction of paraffins is slower than the hydrogenation reaction of the benzene. Furthermore, the lower temperature in the second reactor is thermodynamically favorable to the creation of the desired branched products.
However, the benzene content of the isomerization charges remains relatively limited. Indeed, a rule commonly accepted suggests that a 1% increase of the benzene content in the charge to be treated produces an increase in temperature of 10° C. inside the first isomerization reactor. Knowing that the reactions other than the hydrogenation of benzene already produce an increase in temperature in the range of 15° C. on their own, the benzene content of the charge entering this reactor must then be limited to 4%. Beyond this content, the temperature inside the reactor is too high, which, in the end, not only harms the catalyst but also the unit, built to operate at a limited temperature. Furthermore, when the temperature is high, undesirable secondary reactions occur, such as for example, hydrocracking reactions of the charge.
However, it seems desirable to be able, in the isomerization unit, to treat charges with a benzene content that is much higher than the 4% generally accepted.
Indeed, considering the carcinogenic character of benzene, the current standards have a tendency to impose tougher and tougher limitations on the content of this composition in fuels.
An ingenious solution then consists in allowing, in the isomerization unit, in addition to the traditional charges, gasoline cuts that are rich in benzene, such as certain gasoline cuts resulting from catalytic reforming or cracking units: the benzene that is present in these cuts is thus hydrogenated in the isomerization unit, and this operation allows in fine to clearly reduce the benzene content of said cuts before carrying them to the “gasoline pool”, term by which we designate the overall bases used in manufacturing oil products.
However, this need to treat more benzene during isomerization comes up against the limitation tied to the temperature in the first reactor. Indeed it is still essential that this temperature be maintained at an average value in the range of 180° C., above which the hydrocracking reactions, which are very exothermic, begin.
In order to rid oneself of this limitation, U.S. Pat. No. 5,003,118 proposes to introduce, upstream from the actual isomerization reactor (or reactors), a charge pretreatment reactor, specifically intended to execute the hydrogenation of the benzene present in this charge. In this way, the unit can treat charges with a higher benzene content. However, this solution has the disadvantage of being costly and requires building additional equipment. Furthermore, it is not absolutely flexible to the extent that the hydrogenation reactor is of no use in the case of a charge with a low benzene content.